Method and device of the production of brushes

ABSTRACT

In a method for producing a bristle from thermoplastic polymers through injection molding, the molten polymer mass is injected under pressure into a bristle-molding channel of predetermined length having a predetermined cross-section along this length and the channel is vented during injection molding. To produce injection-molded bristles with excellent bending behavior, the magnitude of the injection pressure is adjusted in dependence on the cross-sectional extension of the bristle-molding channel such that a shear flow is generated with high core speed in the center of the molten polymer mass flow and large shearing effect due to wall friction of the molten polymer mass under distinct longitudinal orientation of the polymer molecules at least in the region of the molten polymer mass close to the wall, which is maintained along the channel wherein the channel is simultaneously vented along its length to support maintenance of the shear flow. A device for carrying out the method is also described.

This application is the national stage of PCT/EP03/00131 filed on Jan.09, 2003 and also claims Paris Convention priority of DE 102 01 635.6filed Jan. 17, 2002.

BACKGROUND OF THE INVENTION

The invention concerns a method for producing a bristle from athermoplastic polymer through injection molding, wherein the moltenpolymer mass is injected under pressure into a bristle-molding channelof predetermined length having a predetermined cross-sectional shapealong this length and the channel is vented during injection molding.The invention also concerns a device for carrying out this method.

Animal hair and natural fibers which were previously used as bristlematerial for producing brushes, paint brushes or the like have beensubstantially replaced by artificial bristles, wherein the production ofthe bristle material is based largely on long-standing technologyrelated to the production of synthetic textile fibers, i.e. extrusion orspinning processes. However, a bristle is subjected to completelydifferent conditions than an endless fiber in a fiber composite. It isfree and fixed at only one end and can be regarded in terms of stabilityas a bar which bends and which is fixed at one end. Pressure orcompression forces and sometimes also tensile forces occur during use.Compared to endless fibers, the production requirements are differentwith regard to bending strength, fatigue strength under reversed bendingstresses, buckling resistance and bend recovery.

Monofilaments for bristles are therefore extruded having relativelylarge diameters up to a few millimeters. Shaping by the extrusion andspinning nozzle produces a certain longitudinal orientation of themolecules in the molten polymer mass which is however not sufficient toprovide the monofilament with the desired properties. The monofilamentis therefore drawn, i.e. stretched under appropriate drawing forces,which usually requires pre-drawing, post-drawing, and subsequentlythermal stabilization, which can be repeated, if required. The endlessmonofilament is subsequently wound up and the wound-up product is againstabilized, if required.

If, for production of brushes, the endless monofilaments are notprocessed directly from the spool—which is still the exception today—alarge number of monofilaments are combined into strands and bound andcut to suitable lengths of between 60 and 120 cm. The strand material isagain cut to a length, which is slightly longer than the final bristlesthereby producing waste of approximately 30% of the initial material.For high-quality plastic bristles, e.g. of polyamides (nylon), which arerequired for quality brushes, e.g. toothbrushes, hygiene brushes etc.,the price for the raw material is the most expensive factor in the brushprice. The price of extruded bristles is consequently considerablyincreased by the large amount of waste.

For brushes, the production of bristles is followed by mounting thebristles to the bristle support, which can be effected eithermechanically or thermally. Since the free length of the bristles largelyvaries in this intermediate state, shearing-off and in most casespost-processing of the bristles and mainly of the bristle ends followsto remove the sharp cutting edges. If the effective brushing surfaceformed by the free ends must meet special requirements, e.g. fortoothbrushes, the brushing surface must either be given a contouralready during mounting or the flat brushing surface must besubsequently shaped, which produces additional waste of approximately10%.

Considering the fact that approximately 90% of the worldwide need ofbristles is limited to bristles having a length of <10 cm, the endlessproduction through spinning including all subsequent work processesuntil the bristle is finished is highly uneconomical, due to the rawmaterial waste alone. Further limitations result from the fact thatmonofilaments can usually only be produced with cylindrical shape andwith profiled cross-section such that the structure of the bristles islimited and extensive later processing may be required.

Injection molding production of brush bodies, brush handles, paint brushhandles etc. from plastic material was established quite early in thebrush and paint brush industry to utilize the numerous structuralpossibilities of injection molding technology. Various attempts weremade to produce the brush body with integral bristles through injectionmolding. In practice, these methods are used only for bristles of thelowest quality and stability requirements, in particular those which areused only once or a few times. Injected bristles have a much worsebending strength, fatigue strength under reversed bending stresses andbuckling strength, insufficient bend recovery and low wear resistance.Injected brushes have highly conical bristles with relatively largecross-sections in the root region of the bristle due to the method, andare therefore more appropriately described as pins or bolts rather thanbristles. Some known injection molding methods in brush technology aredescribed below.

Rotating bristles for grinding and polishing surfaces are composed ofdisc-shaped brush segments, which are produced individually throughinjection molding (U.S. Pat. No. 5,903,951). Each brush segmentcomprises a central support disc from which the bristles outwardlyextend radially or at an angle inclined against the direction ofrotation relative to the radial direction. The brush segments consist ofa thermoplastic or thermoelastic polymer (TP or TPE), which is filledwith abrasive particles. The bristles preferably have a length ofbetween 1 cm and 5 cm and a diameter of between 0.25 mm and 10 mm,preferably between 1 mm and 2 mm. In one concrete embodiment, theconical bristles have a length of 75 mm and a diameter of 2 mm at theroot and 1.5 mm at the tip. The two-part injection mold consists of twoplates having the cavities for the support disc and the bristles onmutually facing sides, which simultaneously form the mold-separatingplane. The molten polymer mass with the admixed abrasive particles isinjected from the center of the support disc at an injection pressure of690 to 6900 kPa (0.59 to 69 bar). The preferred pressure range isbetween 2070 to 4830 kPa. The required venting of the mold cavity occursin the mold-separating plane, i.e. parallel to the bristles. Thisunavoidably produces two mold-separating seams on the bristle jacket,which extend from the root to beyond the tip. The abrasive particlescause additional narrowing of the small cross-sections in the bristlecavities and the molten polymer mass solidifies too quickly at theselocations prior to complete filling of the bristle cavity. For thisreason, injection molding is preferred in two steps, wherein a highlyfilled molten polymer mass is initially injected into the bristlecavities and a more or less unfilled molten polymer mass is thensubsequently injected. One of average skill in the art knows that duringinjection molding, practically no molecular orientation takes place inthe polymer (US 2001/0007161 A1, see column 1, paragraph 0006). Thisproduces a completely insufficient bending behavior for bristles, whichis additionally deteriorated by the admixed abrasive particles. Thestated maximum injection pressure of 6900 kPa (69 bar) is stronglyreduced through the flow resistance in the narrow mold cavity forforming the carrier disc and in the subsequent bristle channels suchthat the person skilled in the art may have reasonable doubts about thepracticability of this method.

U.S. Pat. No. 3,618,154 describes the production of a toothbrush in onesingle injection molding process wherein the bristles on the brush headare injected in a type of bundle arrangement. Towards this end, thetwo-part injection molding tool whose mold-separating plane is in theplane of the bristle head, has substantially cylindrical bores whichextend from the mold surface forming the bristle side of the brush head.Substantially cylindrical mold cores engage in the bores from theopposite side wherein one of their end faces forms part of the moldsurface for the bristle support side of the head and—startingtherefrom—comprise groove-like depressions which extend along jacketlines. These groove-like depressions taper uniformly and conically fromthe front-side mold surface towards the other end and terminate in asemi-spherical dome on the jacket of the mold core on which thedepressions are uniformly distributed. Each depression forms, togetherwith the bore wall in the one part of the injection mold, abristle-molding channel, which consequently conically tapers from themold cavity for the brush head towards the other end. The channels arevented across their entire length in the separating surface between moldcore and bore, i.e. substantially parallel to the bristles. U.S. Pat.No. 3,618,154 requires high precision of the cooperating surfaces. Eachbristle inevitably has two mold-separating seams, which extend alongjacket lines on the bristle. It is also not possible to produce bristleswith circular cross-section since the groove-like depression in the moldcore has a substantially larger radius of curvature than the bore. Thisproduces a cross-sectional shape with discontinuities at which themold-separating seams, which cannot be subsequently removed, immediatelyform. The bending behavior of the bristle differs in differentdirections transverse to its axis. Furthermore, the bundles are notfilled (their center is free) so that the bristles cannot support eachother as is the case in conventional bundles. The serious problem ofremoving the individual bristles from the mold is intended to be solvedthrough corresponding conicity of the bristle-forming grooves. This canobviously not work, since the mold cores are simultaneously used asejector pins which push towards the bristle tips during release from themold via the dome-shaped ends of the groove-like depressions. Theconicity is intended to make the bristle ends relatively flexible duringuse of the toothbrush. This document does not describe any measureswhich extend beyond conventional injection molding technology and whichcould improve the bending behavior of the injection-molded bristles. Inthis case as well, the polymer molecules, as is usual in injectionmolding, have the energetically favorable balled shape, which is,however, unfavorable with regard to stability (US 2001/0007161 A1).

Moreover, in conventional toothbrush production (U.S. Pat. No.5,158,342) the bristle stock is subsequently injected into a prepareddepression of the brush head of a pre-injected brush body, consisting ofhandle and the brush head. This produces bristles of completelyinsufficient bending behavior due to the conventional injection moldingtechnology with injection pressures of 30 to 60 bar (3000 to 6000 kPa).

GB 2 151 971 also describes two-step production of bristle stock and abristle support. In particular, this document clearly illustrates theproblem of releasing the bristles from the bristle-molding channels.Despite the strong conicity of the bristles, which is favorable forrelease from the mold, the mold removal process is slow and highlycontrolled, which impairs the efficiency of the injection moldingsystem. Injection molding measures to increase the bristle stability arenot described.

Much better results are obtained according to an older, notpre-published patent application of the inventor (PCT/EP01/07439) withwhich a bristle support is provided with bores which have a nozzle-likecross-sectional shape. The molten polymer mass for the bristles isinjected through the nozzle-like bores into adjoining molding channelsof an injection mold. This method produces a semi-finished product frombristle support and bristles or also—with corresponding shape of thebristle support—a finished brush, wherein the bristles have bendingbehavior characteristics, which are similar in quality to those ofextruded, bristles. The shape of the bristles is not subjected to theconstraints of endless production of extruded monofilaments.

U.S. Pat. No. 4,712,936 discloses production of small applicationbrushes, e.g. for decorative cosmetics, which are inserted into acontainer and mounted to the sealing cap of this container, as one-partinjection molding part which consists of cap, a stem centricallyadjoining the inner side thereof, and brush bristles disposed at the endthereof. The mold cavities for the cap, the centrical stem and thejoining bristle-molding channels are formed in the two parts of aninjection molding tool with axial orientation, wherein the cap openingis in the mold separating plane thereof. The stem and bristles areproduced through mold cores, which are pushed coaxially into each other.The injection side is on the cap. The molten polymer mass mustconsequently traverse long flow paths with several cross-sectionalchanges and overcome large mass requirements before reaching the thinbristle channels. The entire venting of the stem and bristle regiontakes place at the ends of the bristle channels via a cylindricalclosure with knurl structure which is to form a type of filter with highflow resistance. This prior art shows that the bristles that can beproduced through injection molding are not suitable, in particular, foruse as paintbrushes. After removal from the mold, they are thereforere-heated outside of the injection mold and are subsequently drawn. Thecross-section is thereby reduced which inevitably increases theseparation between the bristles. However, for application brushes ofthis type, the bristles should be disposed at minimum mutual separationsto produce capillary action between the bristles for storing andretaining the application medium.

Attempts have also been made (DE 21 55 888 C3) to produce a brush withformed-on bristles through injection molding with an injection moldingtool having a first tool part for the bristle support and a second toolpart largely covering the open mold cavity in which a short channel isformed which widens at its opposite end and is closed at that location.During injection, the molten polymer mass penetrates from the moldcavity of the support into the short channel and flows into the wideningto produce a short bolt with a head. During opening of the mold, thehead is carried along and the bolt-like bristle blank is drawn. This canproduce a certain molecular orientation, which increases thestability—similar to production of endless monofilaments.

Attempts to replace production of bristles from extruded endlessmonofilaments and their subsequent mounting to separately produced brushbodies, with an injection molding of the entire brush with bristles havetherefore obviously failed (US 2001/0007161 A1).

This is also true for the known suggestion of only producing the bristlethrough injection molding (U.S. Pat. No. 3,256,545). This closest priorart is based on the realization that extruded bristles have ends ofincreased flexibility imparted to them by post-processing of the bristleends as do bristles obtained through injection molding of one-piecebrushes in consequence of the conicity required for injection molding,which have however disadvantageous effects with regard to wearresistance and durability. This patent method suggests improving thewear resistance, which decreases towards the ends, by enlarging thecross-section of the injected bristle, going from the end on themounting side (the injection side bristle root) towards the free end.The cross-sectional shape may increase continuously or discontinuously.In any event, a larger amount of plastic material is present in theregion of the working ends of the bristles than on the mounting sideend. The insufficient properties of known conical bristles arecompensated for through accumulation of a larger plastic mass in theregion of the bristle ends. However, one has thereby overlooked the factthat, as the plastic mass or cross-section increases, the proportion ofthe energetically favorable balled structure increases, i.e. the bristleexcessively looses bending elasticity due to the enlarged cross-section.This injection molding method proposes injection pressures of between800 and 1200 bar (approximately 0.8·10⁵ to 1.2·10⁵ kPa), which arerequired to introduce the molten polymer mass through the channels,which are initially narrower on the injection side, into the extendedchannels such that they fill the mold. Despite the relatively highpressure, the recommended bristle diameters of unoriented molecularstructures are between 1.6 and 2.2 mm in the region of the thinnercross-section and between 11 and 12 mm in the region of the thickercross-section (column 5, lines 43 to 48 and column, lines 32 to 42).Support structures of the same molten polymer mass are formed on theinjection side of the bristles, for mounting the injection-moldedbristles to a bristle support, and interconnect several bristles, ifrequired.

The technical literature also teaches (Ehrenstein: Eingenverstärkung vonThermoplasten im Schmelze-Deformationsprozeβ in the German magazine “DieAngewandte Makromolekulare Chemie” 175 (1990), pages 187 to 203) thatfor polyamides, only 3% and 6% and for polyethylene only 33% and 5.5% ofthe theoretical mechanical values for the modulus of elasticity [N/mm²]and tensile strength [N/mm²] respectively are obtained through extrusionand injection molding methods, wherein for injection-molded components,the tension-free state (molecular balled structure) is preferred.

It is the underlying object of the invention to produce bristles throughinjecting molding whose bending behavior and bend recovery is superiorto that of extruded bristles, and which permits maximum attainment ofthe theoretical values of the modulus of elasticity and tensile strengthto produce bristles of high quality through a large length range withrelatively small cross-sections for simplified production of bristlegeometries and bristle arrangements adjusted to the requirements of thefinal product such as brushes or paint brushes. The invention alsoconcerns a device, which is suitable for carrying out the method.

SUMMARY OF THE INVENTION

Departing from the known injection molding method, wherein the moltenpolymer mass is injected under pressure into a bristle-molding channelof predetermined length and predetermined cross-sectional shape alongthis length, and the channel is vented during injection molding, thisobject is achieved in that the magnitude of the injection pressure isadjusted in dependence on the cross-sectional shape of thebristle-molding channel such that a shear flow of the molten polymermass is generated with high core speed in the center of the moltenpolymer mass flow and large shearing effect due to wall friction underdistinct longitudinal orientation of the polymer molecules, at least inthe region of the molten polymer mass close to the wall, which ismaintained along the channel, wherein the channel is simultaneouslyvented along its length to support maintenance of the shear flow.

The invention Is based on the realization that the bending behavior of amonofilament can be primarily increased through generation andmaintenance of a molecular orientation which has previously not beenrealized in injection molding of bristles, brushes and paint brushes.The molecular structure in a molten polymer mass flow can only besubstantially influenced using sufficiently narrow cross-sections andmelt flow forced to a speed profile having strong shearing effects todeform and stretch the energetically most favorable tension-free balledstructure. For this reason, in accordance with the invention, theinjection pressure is set to a sufficiently high level that a steep flowprofile forms in the bristle-molding channels which is characterized bya high core speed in the center of the flow and large shearing effect inits edge region due to the wall friction of the molten polymer mass onthe channel wall, wherein the shearing forces due to wall friction arelarger the higher the speed difference between neighboring flow layers.A flow profile of this type with high core speed moreover ensuresperfect filling of the mold of the bristle-molding channel even for thenarrowest of cross-sections (small bristle diameter) and large channellength (bristle length).

The speed profile can be set in dependence on the predeterminedcross-sectional shape along the length of the bristle-molding channelthrough a correspondingly high, optionally variable injection pressure.The polymer molecules are thereby oriented longitudinally close to thechannel wall and, to a reduced degree, within the entire melt flow,wherein the magnitude of the core speed moreover prevents prematuresolidification of the molten mass, even for small cross-sections andlarge lengths.

High pressure alone is not sufficient for rapid filling of a narrowmolding channel. In accordance with the invention, the channel is ventedalong its length such that the shear flow with high flow speed ismaintained up to the end of the channel and the desired longitudinalorientation of the molecules reaches the bristle tip.

Practical tests have shown that the injection pressure should be atleast 500 bar (0.5·10⁵ kPa) and is a function of the cross-sectionaldependence of the bristle-molding channel. For the quality bristlesunder discussion having an average bristle diameter of e.g. 0.3(measured at half the length) and a corresponding cross-section of thebristle-molding channel and with a length of 10.5 mm, the desired speedprofile can be produced with an injection pressure of at least 500(0.5·10⁵ kPa). Approximately ⅔ of the above-mentioned injection pressurecan usually be converted into specific pressure in the bristle-moldingchannel such that the molten polymer mass in the channel should have apressure >300 bar (0.3·10⁵ kPa).

During solidification below the crystal melt temperature, thermoplasticmaterials form crystallites, which influence the modulus of elasticity(E module) and the tensile strength (tearing strength) in dependence ontheir shape and configuration. The formation of needle crystals has apositive influence on stiffness through increase of the E modulus andstrength due to an increase of the tensile strength and initiallyrequires linked elongated crystal seed formation on parallel molecularsections. This seed formation can be amplified compared to isothermalcrystallization through the introduction of forces as given i.a. in flowprocesses. The inventive high injection pressure and the high flowvelocity of the molten polymer mass in the bristle-molding channelobtained thereby therefore not only promote longitudinal molecularorientation but also crystal formation, wherein the high pressuresimultaneously increases the packed density of the crystals throughincreased loading. The partial crystallization of the molecularlyoriented molten mass increases the relaxation time, i.e. the molecularorientation lasts for a longer period.

The above-described effects are further supported in a supplement to theinvention in which the bristle-molding channel is cooled.

The narrower the cross-section and the larger the length of thebristle-molding channel, the more reasonable it is to keep the channelwalls warm to maintain the viscosity of the molten polymer mass andobtain complete filling of the mold. When setting the inventive methodparameters, the filling of the mold is also guaranteed when thebristle-molding channel is cooled. Cooling of the channel and associatedintroduction of forces additionally promote formation of crystals andincrease relaxation time. The stabilizing outer layer of the bristle,which is produced on the channel wall, permits increase of thepost-pressure, which is common in injection molding. The higher thepost-pressure, the stronger the crystal seed formation in the stillmolten bristle core. The pressure simultaneously increases the meltingtemperature and enhances cooling of the molten mass for a given masstemperature, thereby further producing a positive effect on the crystalgrowth speed and impeding relaxation of the molecules.

The high injection pressure and high flow velocity require particular oradditional measures for rapid and effective venting to ensure completemold filling and to prevent cavities in the molding channel or airinclusions in the molten mass. In the conventional injection moldingmethods, the bristle-molding channel is vented when the cavity iscompletely closed at the end of the channel or, for a longitudinallysplit injection mold defining the channel, in two planes parallel to thebristles. In the first case, for forming a perfect, preferably roundedbristle end, the venting must be drastically reduced to prevent moltenpolymer mass from getting into the venting regions. For venting parallelto the bristles, the mold-separating plane lies in the flow directionwith the consequence that the molten polymer mass penetrates into eventhe most narrow of venting gaps and produces mold-separating seams alongthe bristle jacket.

The invention therefore proposes venting of the bristle-molding channeltransverse to the flow direction of the molten polymer mass, wherein theventing is preferably effected in several planes transverse to the flowdirection of the molten polymer mass. The number of venting planes ishigher, the longer the bristle-molding channel such that, forpredetermined channel length, the venting is controlled in dependence onthe speed of the molten mass front. Since venting is possible in such aplane about the entire periphery of the bristle channel, there is acorresponding gap length transverse to the flow direction which islarger than that of a bristle-parallel mold-separating plane and whichcan be implemented over a plurality of planes.

The venting planes can be provided at equal separations along the lengthof the bristle-molding channel in dependence on the volume to be vented,optionally with progressive or degressive separation in the flowdirection of the molten polymer mass. This permits simultaneousmaintenance of a sufficiently high counter pressure in the channel toobtain uniform filling of the mold.

The bristle-molding channel can be vented merely through displacement ofair through the flow pressure of the molten polymer mass. However,venting can also be supported by external underpressure.

The inventive method permits injection of the molten polymer mass into abristle-molding channel from the injection side with a cross-section,which is substantially uniform to produce a substantially cylindricalbristle, which could not be produced with previous injection moldingtechnology for bristles and brushes.

The cross-section may substantially continuously taper from theinjection side to produce a bristle with preferably only weak conicity,which is desired for many applications to increase the bendingelasticity from the bristle root to the bristle end. Such conicitypromotes maintenance or even reinforces a steep velocity profile withhigh core speed and shearing effect in the edge region which increasesalong the length such that, despite increased flow resistance, themolecular orientation and crystal formation is enhanced towards thebristle end.

Injection molding produces precisely sized bristles with a tolerance of±3% in cross-section and in length while extruded bristles with the sameconstructive parameters have tolerances of ±10%. The initially circularcross-section of extruded bristles is ovalized through processing whichis unnecessary for the bristles produced according to the presentinvention.

Injection molding technology usually regards mold removal slopes of afew degrees (>1.00°) as necessary to be able to properly remove theinjection-molded part. Mold removal is usually supported by ejectors.When the bristles are injection-molded in accordance with theabove-mentioned prior art, the mold slope must be considerably larger toprevent tearing off of the bristle during removal from the mold (U.S.Pat. No. 3,256,545). This is one reason why prior art usesinjection-molding tools, which have a bristle-parallel mold-separatingplane, thereby accepting the above-described disadvantages. Theinventive method permits reduction of the mold slope to a value of 0°with sufficient mold filling. Slender bristles of great length can beproduced with relatively small conicity in the region of 0.2 to 0.5°when the positive-properties of a conical bristle are desired having abending angle which increases towards the bristle end. Mold removal issimplified by crystal formation promoted by the longitudinal orientationand the associated increase in the tensile strength (tear resistance) ofthe bristle, in particular in the region close to the wall, which isimportant for removal from the mold. Further measures for facilitatingremoval from the mold are described in connection with the device.

In a further embodiment of the inventive method, the molten polymer massis injected into an inlet region which narrows like a nozzle towards thebristle-molding channel for generating an extension flow to produce abristle with a widened root region which optionally tapers continuouslytowards the actual bristle.

Such narrowing generates an extension flow, which produces considerablemolecular orientation and, due to flow properties, correspondingboosting of the flow profile after the narrowing. The narrowing istherefore preferably disposed close to the injection side. It is alsopossible to provide narrowings along the length of the bristle-moldingchannel to obtain stepped bristles wherein, in this case as well, thenarrowings have positive effects on the molecular structure and crystalformation.

After an optional upstream inlet region, the cross-section of thebristle-molding channel is preferably selected with a maximum width of≦3 mm such that the injection-molded bristle has a correspondingdiameter with an optionally broader root region. Bristles having thiscross-section and broader root region cannot be produced throughextrusion or spinning. The term “largest width” in this connection meansthat the bristle may also have a cross-section, which differs from acircular shape, e.g. oval, wherein the largest width of the lengthcorresponds to the larger axis of the oval.

In applications of the inventive method, the ratio between the largestwidth and the length of the channel may be selected to be ≦1:5 to1:1000, preferably up to ≦1:250. Bristles can e.g. be produced whichhave a length of between 15 mm and 750 mm with a maximum diameter of 3mm in or close to the root region. The smaller the largest width, theshorter the length. For stringent requirements, e.g. for toothbrushes,application brushes etc. diameters above the root region of ≦0.5 mm arerecommended which permit bristle lengths of more than 60 mm in theinventive method.

The inventive method can be modified in a likewise advantageous fashionwhen the molten polymer mass is injected simultaneously into severalneighboring bristle-molding channels thereby forming a correspondingnumber of bristles such that a set of bristles can be produced in oneinjection process. Minimizing of the separation of the bristle-moldingchannels produces bristle arrangements in the form of pucks throughslight compacting of the removed bristles.

The number and arrangement of the bristle-molding channels can beselected such that the entire bristle stock of a brush or of a paintbrush is produced in one injection process, wherein the separationsbetween the bristles and their geometrical relationships can be variedin accordance with the desired arrangement in the bristle stock.

A further embodiment provides that the molten polymer mass is injectedinto the neighboring bristle-molding channels thereby simultaneouslyforming a connection between at least two bristles, wherein theconnection may serve for further handling of the connected bristles andalso as an aid for connection to a brush body, paint brush handle or thelike. Alternatively, after injection of the bristles from a polymer, amolten polymer mass of another polymer can be subsequently injected toproduce a connection between the bristles. The connection may be in theform of bars, grids connecting several bristles; or the like. The use ofdifferent polymers with a joining factor of ≧20% guarantees sufficientlysecure connection.

The connection may further be designed such that it forms a bristlesupport which may simultaneously constitute the brush body or partthereof or which can be completed into a brush body or paint brushhandle by injecting at least one further molten polymer mass which maycomprise a different thermoplastic or thermoelastic polymer.

In a further variant of the method, several bristles can be injectedwith different lengths such that, in combination with the bristlesupport connecting them, a complete bristle stock or partial stock canbe produced for a brush or a paint brush, wherein the bristle ends areat different heights along a flat or non-planar envelope surface so thatthe finished brush has optimized bristle end contours.

The bristle group can also be injected with different cross-sections topermit different effects in predetermined regions of a finished brush.

Likewise, the bristle group can be injected with a cross-sectionalshape, which differs along its length. The bristle group can also beinjected in a mutually non-parallel fashion to produce a bristle stockwith differing bristle orientations.

In accordance with another embodiment of the method, bristles having thesame geometry but different bending elasticity (hardness) can begenerated through injection molding of different molten polymer massesin the same molding channels. For extruded bristles-for brushes havingdifferent degrees of hardness (textures) e.g. for toothbrushes havinghardness gradations of soft, medium, hard, the desired degree ofhardness could be influenced only via the diameter of the bristle, i.e.toothbrushes of the same structure had to prepare and process up tothree different bristle diameters. The inventive method realizes thesedegrees of hardness merely through selection of the polymer andoptionally by adjustment of the injection pressure but with identicalbristle diameters.

Bristles can also be injected from a polymer or a polymer mixture, whichhave reduced secondary binding forces in the solidified state. Thesebristles can be cleaved after production through mechanical forcesthereby forming flags, if necessary only after further processing intobrushes or paintbrushes.

The bristles can be injected from a polymer comprising additives, whichbecome active during use. The additives may have mechanical, e.g.abrasive effect or, e.g. for toothbrush bristles, be additives withprotective, therapeutical or remineralizing action. Numerous additivesof this type are known.

The invention also concerns a device for injection-molding bristles fromthermoplastic polymers, comprising a means for producing the injectionpressure and an injection mold which has at least one supply channel forthe molten polymer mass and at least one cavity in the form of a moldingchannel with a mold contour which corresponds to the length andcross-sectional shape of the bristle to be produced, wherein the moldingchannel has associated venting means for releasing the air displacedduring injection molding. Devices of this kind are known from theabove-described prior art.

A device of this type is characterized, in accordance with theinvention, by means for generating an injection pressure of preferablyat least 500 bar (0.5·10⁵ kPa) and the venting means have ventingcross-sections which are distributed along the length of the moldingchannel and which are designed to form, in cooperation with theinjection pressure, a shear flow with high core speed in the center ofthe molten polymer mass and large shearing effect on the wall of themolding channel.

Such a device can produce bristles through injection molding asdescribed in connection with the method. Compared to known injectionmolding devices for producing bristles or one-piece brushes withbristles, the device according to the invention is designed such thatthe desired flow dynamics is obtained in the channel forming thebristle.

The means for generating the injection pressure is preferably designedsuch that injection pressures of between 500 and 4000 bar (0.5·10⁵ to4·10⁵ kPa) can be set depending on the length and cross-sectional shapeof the molding channel. The pressure is higher, the smaller thecross-section of the bristle to be produced and the greater its length.

The means for generating the injection pressure and ventingcross-sections on the molding channel are designed with respect toconstruction and control such that the molten polymer mass in themolding channel has a specific pressure of at least 300 bar (0.3·10⁵kPa) to 1300 bar (1.3·10⁵ kPa). This design is adjusted to the mass flowand flow resistances to be overcome upstream of the molding channel.

If the injection pressure on the generating means is sufficiently high,the injection pressure can advantageously be controlled depending on thelength and the cross-sectional shape of the molding channel to permitinjection of injection molds of different geometrical shapes with oneinjection-molding unit.

This purpose is supported in that the venting means have ventingcross-sections, which can be controlled depending on the specificpressure.

In the inventive device, the injection mold is advantageously associatedwith coolant, which may be external cooling after each injection moldingcycle or after removal from the mold. The molding channel in theinjection mold may have associated cooling means for keeping the moldingchannel at a reduced temperature.

In a particularly preferred embodiment of the invention, the injectionmold consists of several molding plates disposed in layers transverse tothe longitudinal extension of the molding channel, each of which definesa longitudinal section of the molding channel.

In contrast to prior art with more or less block-shaped injection molds,the invention provides a structure of stacked molding plates. Thisstructure permits forming of minimum bore cross-sections with highprecision in each molding plate of low thickness. This and any otherproduction technology would fail for larger bore depths. This is also areason why longitudinally separated injection molds were necessary forthe production of narrow cross-sections. Their disadvantages aredescribed in connection with prior art. The inventive decomposition ofthe injection mold into several plates permits realization of moldingchannels of large length with high and reproducible precision over theentire length. The molding plates which comprise the end of the moldingchannels and form the bristle end can have, due to the small thicknessof the molding plates, cavities with only small depth to form a bristleend having clear contours, without any mold separating seam, and withoutadditional venting means. Oxidation of the polymer, which can beobserved in narrow mold cross-sections through the so-called dieseleffect, does not occur due to the small depth of the cavity.

The layered structure of the injection mold moreover permits formationof the venting means on the molding plates, i.e. with a frequencycorresponding to their number. The venting means are preferably formedbetween the mutually facing support surfaces of the molding plates e.g.through narrow gaps or channels. The high flow velocity of the moltenpolymer mass perpendicular to such narrow gaps or channels prevents themolten mass from penetrating into the venting openings, despite the highpressure. The venting openings may therefore be larger than in atwo-shell mold whose mold-separating plane is in the flow direction ofthe molten mass. The venting cross-sections may be formed with a maximumwidth of only a few μm up to 300 μm.

The venting means are preferably formed completely or partially throughsurface roughnesses on the mutually facing surfaces of the moldingplates.

In a further advantageous embodiment, the venting means have ventingcross-sections which increase outwardly from the surface of the moldingchannel such that the air can freely escape after passage of thenarrowest point of the venting cross-sections.

The displacement of air caused by the specific pressure in the moldingchannel can be supported when the venting means is connected to anexternal underpressure source.

The device may be designed such that the molding channel has across-section which is substantially constant along its length or whichsubstantially uniformly tapers towards its end to produce cylindrical orslightly conical bristles.

Practical injection tests under the stated method conditions have shownthat the molding channel can taper at an angle <1.0°, with linear axis,to produce sufficient mold slope for removing a slightly conicalbristle, having excellent bending behavior, from the mold.

The molding channel can have a cross-section, which discontinuouslytapers towards the end to produce specially designed bristle ends asrequired by the application for the finished brush.

The largest width of the cross-section of the molding channel ispreferably ≦3 mm. This covers the bristle cross-sections desired forquality brushes and paintbrushes.

At least one molding plate can be disposed on the injection side havinga widening which tapers towards the molding channel and can be connectedupstream of the molding plates defining the molding channel having theabove-mentioned largest width at their sides facing the supply channelto reinforce the cross-section on the bristle root and on the bristlebase and also to obtain, due to this widening, an extensional flow atthe inlet region of the molding channel to support formation of thedesired flow dynamics. The widening can narrow like a trumpet towardsthe molding channel to produce a smoothly connecting shoulder at thebristle and to the support connecting the bristles, brush body or thelike. This is particularly important for hygiene brushes of any type.

The ratio between the largest width of the cross-section of the moldingchannel and its length is preferably between 1:5 and 1:250 but may alsobe 1:1000 wherein the ratio is closer to the higher value the narrowerthe cross-section of the molding channel and closer to the lower valuethe larger the narrowest cross-section.

A further embodiment of the invention provides that the number andthickness of the molding plates is matched to the length of the moldingchannel, wherein the number of the molding plates is inverselyproportional to the ratio between the largest inner diameter of thecross-section and the length of the molding channel. The number ofmolding plates, which belong to an injection mold, can be variable to beable to produce bristles of varying length with the same mold.

The molding plates preferably have a thickness, which is approximatelythree to fifteen times the central diameter of the molding channel. Fora bristle of an average diameter of 0.3 mm and a length of 10.5 mm, themolding plates have e.g. a thickness of 1.5 mm to 2.00 mm. Alongitudinal section of the molding channel of 1.5 mm to 2.0 mm can bedrilled with high precision into the molding plate.

The molding plates are movable perpendicular to their plate plane,individually or in groups. This permits, in particular, removal of thebristle from the mold in a non-conventional fashion, wherein e.g. themolding plates, starting with the molding plate having the mold contourat the end of the molding channel and ending with the molding platefacing the supply channel, can be subsequently removed eitherindividually or in groups.

The molding plates are reliably kept together under the method-specifichigh closing pressure of the injection-molding machine and are notsubjected to any deforming forces, despite their low thickness.Moreover, the venting openings are kept closed by the closing pressureand, unlike channels with longitudinal venting, require no additionalmeans to keep them closed.

Practical tests have shown that the inventive narrow cross-sections andchannel lengths require considerable extraction forces to release thebristles if e.g. only two molding plates are present. The bristleusually breaks. Increasing the number of plates and their successiveseparation from each other permits damage-free removal of the bristlefrom the mold, in particular when the molding plate facing the supplychannel is removed last. During removal from the mold, the edges of theholes of each molding plate function as drawing nozzles to flatten any“polymer skin” formed in the mold-separating plane withoutdisadvantageously effecting the bristle jacket. In any event, thebristle ends are perfectly formed.

Individual molding plates may be displaceable parallel to theneighboring molding plates to exert transverse loading on the bristleafter injection molding, thereby optimizing the molecular structure.

In a further preferred embodiment, the injection mold has moldingchannels of different length and/or different cross-sectional shape toobtain e.g. a bristle stock of the desired geometry and configuration inone injection molding cycle.

In accordance with a further embodiment, the injection mold has moldingchannels comprising a central axis which extends at an inclined anglerelative to the direction of motion of the molding plates, wherein eachmolding plate comprises a longitudinal section of the molding channelwith a length which permits removal from the mold through successiveremoval of the individual molding plates, despite the angle variation.

The subdivision of the injection mold into a plurality of molding platesextending transverse to the molding channel permits subdivision of themolding channel into longitudinal sections which nevertheless permitremoval of the individual longitudinal sections from the mold withoutexcessive strain on the bristle or deformation thereof even when thebristle axis is inclined relative to the direction of motion of themolding plates (removing direction). In this fashion, bristle groups canbe produced in one single injection mold, wherein the bristles extendparallel to each other but at an angle relative to a bristle supportconnecting them or which have different angle orientations relative toeach other.

In accordance with a further embodiment, the injection mold has moldingchannels with a central axis which is curved relative to the directionof motion of the molding plates, wherein each molding plate defines alongitudinal section of the molding channel which is dimensioned suchthat removal form the mold is possible through successive lifting ofindividual molding plates in dependence on the curvature.

Wavy bristles can thereby be produced which can also be easily removedfrom the mold. It is also possible to simultaneously produce straight,wavy and curved bristles in one single injection mold.

In a further embodiment, the injection mold has at least one moldingplate which can be displaced in its plane relative to the neighboringmolding plates after injection-molding of the bristles to form, togetherwith these, a clamping means for all the bristles which acts on thecorresponding portion of the length of the molding channel.

The invention thereby permits use of parts of the injection mold tograsp the injected bristles and fix them in the injection mold along aportion of their length e.g. to separate the molding plates close to theends, in the removal direction, from the remaining molding plates and tocarry along the bristle blanks such that the bristles are exposed alonga middle partial length, i.e. between these molding plates and theremaining molding plates. Subsequent displacement of the clampingmolding plates and return of the molding plates close to the end in thedirection towards the injection end of the bristles, causes these endsto project past the molding plate at the injection side. Throughtransfer of the injection mold, optionally under further clamping by theholder, the injection mold can be connected to another injection moldingtool, which has a mold cavity forming a bristle support or brush body.In a further injection molding process, the projecting ends aresurrounded by a further molten polymer mass, which fills this moldcavity.

The clamping means may also serve as a transport holder to transfer theclamped bristles, after removal from the other molding plates, intoanother work station for connection to a brush body. This is alsopossible when the bristles are already joined via a connection such ase.g. bridges, grids or bristle supports. The clamping molding plate isthen located close to the transition between bristles and bristlesupport and the holder is removed in the removal direction along withthe connection and subsequently transferred, wherein the molding plateswhich serve as holders are replaced by an equivalent set of moldingplates to again obtain a complete injection mold. The holder can be aportable holder moving in a circulating path and be reused aftercomplete removal of the bristles from the holder to complement theinjection mold. If the connection is not directly required for thesubsequent fabrication steps, e.g. insertion, gluing, welding, injectingetc. it can also be removed and only the bristles may be connected tothe bristle support or brush body using any conventional joiningtechnique.

A further embodiment of the invention provides that the injection moldconsists of at least two groups of molding plates comprising clampingmeans of which the first group comprises part of the molding channelincluding the end and the further groups comprise the remaining part ofthe molding channel, wherein the first group can be removed from thesecond group and the subsequent groups can be removed from each other,in temporal sequence. The injection process is divided into a number ofinjection molding cycles corresponding to the number of groups suchthat, in the closed initial position of the injection mold, the moltenpolymer mass is injected in a first injection cycle into the completemolding channel, whereupon the first group can be removed from thefurther groups thereby carrying along the blank via the clamping means,with the withdrawal path being shorter than the length of the blank.Subsequently, in a second injection molding cycle, more molten polymermass is injected into the released longitudinal section of the moldingchannel of the further groups and the steps injection/removal arerepeated until the second to last group is removed from the last groupto produce bristles of a length greater than the length of the moldingchannel. The bristle is produced in sections, which permits productionof bristles of greater lengths.

In this embodiment of the device, a different molten polymer mass can beinjected in each injection cycle to produce a bristle which has severalcomponents along the bristle length, wherein the polymers used in eachstep can be matched to the requirements of the bristle and connection tothe bristle support thereby producing a bristle with several regions.The removal motions of the individual groups can be matched, in shorttime intervals, to the injection molding cycle, wherein the blank issufficiently cooled that it is removed from the remaining molding platesduring the withdrawal motion. The individual regions are preferablybonded together but may also be connected in a positive or non-positivemanner through corresponding profiling of the end of the last injectedpartial length.

The molding plate comprising the bristle end and the mold contour at theend of the molding channel can preferably be replaced with a moldingplate having a different mold contour for producing bristles with endsof different shapes. This molding plate should only have smooth contoursto permit faultless removal from the mold of the bristle end, which isimportant for the respective use.

In this fashion, the end contour of the bristles can be varied forotherwise constant geometry of the bristles, e.g. have pointed orvariably rounded ends or even to produce bristles with forked-ends (twotips or the like). This molding plate may have longitudinal moldingchannel sections of different depths to form a contoured envelopesurface for the bristle ends of a bristle stock.

A mold cavity, which connects two or more molding channels, ispreferably disposed between the supply channel and the molding channelsof the injection mold for forming a connection among the bristles whichcan optionally also connect all bristles. It can serve either as anauxiliary means for further handling of the entire bristle stock or asan auxiliary means for completing the bristle stock with a brush body.

The mold cavity can also be designed to produce a brush or paintbrushbody or part thereof.

In particular, the mold cavity can thereby be formed from differentpolymers for producing a brush or paintbrush body or part thereof in amultiple component design.

BRIEF DESCRIPTION OF THE DRAWING

The invention is described below by means of diagrams and embodiments.

FIG. 1 shows a diagram of the speed profile in molding channels ofdifferent diameters;

FIGS. 2 to 4 each show a schematic view of an embodiment of the moldingchannel with the respective speed profiles;

FIG. 5 shows a schematic view of a bristle injection-molded in a moldingchannel of FIG. 2 with the speed profiles essential for the longitudinalorientation;

FIG. 6 shows a schematic view of a constriction in a molding channelwith an extension flow;

FIG. 7 shows a schematic view of a conical bristle on a scale of 2:1with dimensions;

FIG. 8 shows a schematic view of a conical bristle on a scale of 1:5with dimensions;

FIG. 9 shows a comparative schematic representation of the speedprofiles in an extrusion nozzle and in a molding channel;

FIGS. 10 through 13 each show a schematic longitudinal section throughan embodiment of an injection mold in different operating phases;

FIG. 14 shows a schematic longitudinal-section through a furtherembodiment of the injection mold;

FIG. 15 shows an enlarged detail of the injection mold of FIG. 14 in theregion of an externally disposed molding channel;

FIGS. 16 through 20 each show a schematic longitudinal section of amodified embodiment of an injection mold in different operating phases;

FIGS. 21 through 23 each show a schematic longitudinal section of afurther embodiment of the injection mold in different operating phases;

FIG. 24 shows a longitudinal section of the injection mold correspondingto FIGS. 21 to 23 with a supplementary mold;

FIGS. 25, 26 each show a longitudinal section of an injection mold in afurther modification and in two operating phases;

FIGS. 27, 28 each show a longitudinal section corresponding to FIGS. 25,26 with a contoured thrust plate;

FIGS. 29, 30 each show a longitudinal section corresponding to FIGS. 25,26 with another blank mold;

FIG. 31 shows a schematic longitudinal section through an injection moldfor producing bristles of different longitudinal extension;

FIG. 32 shows a schematic section through an injection mold forproducing bristles with branched bristle ends;

FIG. 33 shows a highly enlarged schematic view of a bristle;

FIG. 34 shows a highly enlarged schematic view of the arrangement of twobristles;

FIG. 35 shows a highly enlarged schematic view of a further embodimentof the bristle;

FIG. 36 shows a top view onto the free end of the bristle of FIG. 35.

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 schematically shows the flow profile (speed profile) inbristle-molding channels of different diameters. The walls of thechannels are indicated with broken vertical lines and the associateddiameters are given in (mm) below the diagram. The smallestbristle-molding channel has a diameter of 0.3 mm, the largest has adiameter of 6 mm. A constant flow speed in the center of the channel(core speed) leads to the illustrated flow profiles in dependence on thechannel diameter (bristle diameter) which have, in rough approximation,a parabolic dependence. If the diameter of the molding channel remainsconstant along its length, the flow profile does not change its shape ordoes so only slightly.

If the molding channels have a weak conical shape, as schematicallyshown in FIGS. 2 through 4, the core speed can even be increased forconstant pressure and a strong shearing effect can be produced by thewall friction in the region close to the wall. If such a molding channelis loaded with molten polymer mass during injection molding, themolecules experience a strong longitudinal orientation in the wallregion due to the shearing effect while the molecules in the molten masswhich is not subjected to loads have their energetically most favorable,balled structure. For molten polymer mass injected in the moldingchannel under corresponding high pressure, this leads to a strengtheningof the produced bristle in the region close to the wall which extends tothe bristle end for sufficiently high core speeds, with the molecularorientation decreasing towards the center. The molecular orientation dueto shear flow with strong shearing effect in the region close to thewall is also accompanied by tension-induced crystal formation, whereinthe strong shearing effect in the edge region promotes formation of longneedle crystals. Moreover, use of high injection pressure has afavorable effect on the seed formation and the crystal density. With aspecific injection pressure in the molding channel >300 bar (0.3·10⁵kPa), preferably >1300 bar (1.3·10⁵ kPa), the modulus of elasticity andtherefore the bending elasticity can be considerably improved when theforming channel is sufficiently vented, thereby increasing the tearresistance (tensile strength). This specific pressure requires aninjection pressure of >500 bar (0.5·10⁵ kPa) from thepressure-generating means.

A bristle in accordance with FIG. 5, produced in a molding channel ofFIG. 2 has a relatively stiff root region a and a bending elasticityalong its free length l, which increases towards the bristle end as wellas high tensile strength. While the root region a serves mainly forconnection to or embedding in a bristle support or brush body, thebristle has, along its free length l, a stem section which consists of astem base b and the actual stem c. Reduction of the cross-section in theregions b and c, which is essential for the bending deflection, iscompensated for through increase in the bending elasticity due to theabove-described effects. The stem region b, c is joined by the actualeffective region d, i.e. the region important for the brushing effectwhich forms, together with the tip region t, the region which determinesthe flexibility of the bristle. The tip region and its shaping determinethe direct surface effect of the bristle, the penetration depth into thesurface irregularities etc. In contrast to FIG. 2, the bristle can havea trumpet-shaped root region of greater or lesser distinction when awidening is connected upstream of the actual molding channel as shown inFIGS. 3 and 4.

The stabilizing effects can be further improved and, in particular, alsoobtained for short bristle lengths when a discontinuous constriction isprovided on the inlet side of the molten polymer mass before transferinto the actual bristle-molding channel (see FIG. 6). An extension flowis formed at the constriction, which produces a high core speed along ashort path with large shearing effect in the region close to the wall.

The inventive operating parameters for injection pressure and theachievable high core speeds with large shearing effect through wallfriction produce thin bristles of adjustable length using injectionmolding, which has not been possible up to now, not even with extrusionof endless monofilaments, wherein even weak conicity of bristles of suchendless monofilaments can be realized only through considerabletechnical effort (interval withdrawal). FIGS. 7 and 8 show twoembodiments. FIG. 7 shows (scale 2:1) a bristle of a diameter of 0.77 mmin the root region and 0.2 mm at the bristle end, which has an averagediameter of 0.49 mm at half-length. With an extremely weak conicityangle of 0.27°, which corresponds to the mold slope of thebristle-molding channel, bristles of a length of 60 mm or more can beinjection-molded as are required e.g. for high-quality paint brushes orthe like. They have an average diameter at half bristle length ofapproximately 0.5 mm. FIG. 8 shows (scale 5:1) a bristle of a diameterof 0.35 mm in the root region and of 0.25 mm at the bristle end with abristle length of 10.5 mm and the same conical angle (mold slope). Theaverage diameter is 0.3 mm. Bristles of this type are suited e.g. fortoothbrushes. Due to the slender geometry of such bristles, they can bedensely arranged without producing excessive separation in the region ofthe bristle ends—in contrast to conventional injection-molded bristles.

FIG. 9 shows the superiority in terms of technical properties andapplications, of the bristle produced according to the inventioncompared to a bristle produced by extrusion.

During extrusion spinning of a monofilament for producing a bristle ofan average diameter of 0.3 mm, the spinning nozzle has an outletdiameter of 0.9 mm (outer vertical lines in FIG. 9). The molten polymermass has a maximum flow speed (core speed) inside the nozzle oftypically approximately 300 mm/s, which is determined by the extrusionpressure and the withdrawal speed of the monofilament. The monofilament,which leaves the nozzle, is drawn along a short path, by means of thewithdrawal forces, to a diameter of between 0.9 and 0.3 mm and cooleddirectly thereafter to fix the molecular structure. During subsequentdrawing, the monofilament is given its final diameter of 0.3 mm with adiameter tolerance of approximately ±10%. The speed profile isdesignated as e (extrusion) in FIG. 9.

In the inventive injection molding, the bristle-molding channel has anaverage diameter of 0.3 mm (the two inner vertical boundary lines inFIG. 9). An injection pressure in the region of 2000 bar (2·10⁵ kPa)produces a core speed of approximately 1000 mm/s in the channel. Thespeed profile is designated as i (injection). The shearing effect in theflow, in particular in the region close to the wall is relevant for theintrinsic strength of the thermoplastic polymer, which is determined bythe shearing rate (shearing moment) γ. The shearing rate y across theradius r of the flow channel depends on the derivative of the speedprofile with respect to the radius r

${\gamma(r)} = {{{{\mathbb{d}{v(r)}}/{\mathbb{d}r}}} = {\frac{2v_{\max}}{R^{2}} \cdot r}}$which is inversely proportional to the square of the effective diameterof the flow channel. The shearing rate is linearly proportional to themaximum flow speed (core speed). In the above-described example shearingrates for the injected bristle are produced which exceed the statedextrusion flow by at least a factor of 10.

The broken lines in FIG. 9 illustrate the shearing rates without scalingfor extrusion (e₁) and for injection molding (i₁). They have respectivemaxima at the walls of the nozzle of the bristle-molding channel.

FIGS. 10 to 13 schematically show an embodiment of an injection mold indifferent operational phases which is particularly suited for injectionmolding of the bristles according to the inventive method. The scale ishighly enlarged to show the details more clearly.

The injection mold 1 has several long parallel molding channels 2 whichare joined to an injection molding means via a supply channel 3. Theinjection molding means is designed to produce injection pressures inthe region of 500 bar (0.5·10⁵ kPa), preferably >2000 bar (2·10⁵ kPa).The exact magnitude of the injection pressure is set in dependence onthe cross-sectional shape of the molding channel 2 along its length andin dependence on the length itself such that a specific pressure>300 bar(0.3·10⁵ kPa) occurs in the molding channel.

The injection mold consists of a plurality of layered molding plates 4of substantially identical thicknesses, of a molding plate 5 on theinjection side, and a molding plate 6 forming the bristle ends. Eachmolding plate 4, 5 and 6 generates one longitudinal section of themolding channel 2, which is preferably produced by bore holes.

The molding plate 5 has openings 7 on the injection side which narrowtowards the molding channel 2 to produce e.g. the extension flow of FIG.6 and form the root region a (FIG. 5) of the bristle. The subsequentlongitudinal sections of the molding channel in the molding plates 4have a cylindrical or slightly conical cross-sectional shape along theirlength while the molding plate 6 forming the bristle ends has blindholes 8 which are dome-shaped in the embodiment shown.

During injection molding, the molten polymer mass enters into thenarrowing openings 7 of the molding plate 5 via the supply channel 3and, due to the high core speed, fills the entire molding channel up tothe plate 6 forming the ends. The molten polymer mass has asubstantially unordered, balled molecular structure in the supplychannel 3 which is transformed into a longitudinal molecular structurein the opening 7 on the injection side and subsequent molding channel 2due to the strong shear flow.

The molding plates 4, 5 and 6 can be moved perpendicular to the plane ofthe plate to release the injection-molded bristles when they haveachieved sufficient shape stability. The injection molding tool 1 ispreferably cooled such that the wall of the molding channels 2 remainsrelatively cold, thereby supporting the formation of crystals in themolten polymer mass.

To release the bristles from the mold, the molding plate 6 is initiallyremoved (FIG. 11). Only very small adhesive forces must be overcomethereby ensuring that the bristle ends, which are particularly importantfor later use of a brush or a paintbrush, maintain their shapes. Themolding plates 4 are subsequently removed individually or in groups(FIG. 12) until the ends 10 of the bristles 9 are released along most oftheir length. During these releasing steps, the bristles are retained bymeans of the molding plate 5 on the injection side and this moldingplate 5 is also subsequently removed to expose all bristles 9 with theirslightly thickened root region 11 (FIG. 13). The molten polymer mass inthe supply channel on the injection side also effects a connection 12among all bristles 9 and the overall blank can be removed and finishedinto a brush, a paint brush or the like, wherein the connection iseither integrated into the structure or only serves as auxiliary meansfor handling the bristles and is separated off before connecting thebristles to a brush body or the like.

Optimum venting of the molding channels must be provided duringinjection molding to facilitate the desired high core speed. FIG. 14shows an embodiment thereof. Venting occurs via narrow gaps 13 betweenthe molding plates 4, 5 and 6 so that the air is removed along theentire length of the molding channels 2 as the front advances. Insteadof narrow gaps 13, it is also possible to roughen the mutually facingsurfaces of the molding plates 4, 5 and 6, to obtain overall ventingcross-sections of sufficient size. The venting cross-sections havewidenings 14 towards the outside to permit rapid escape of theexhausting air.

The molding channels 2 may taper along their entire length with a moldslope <1.0°, wherein the tapering is not dictated by release from themold but rather by the desired bristle shape and its bending behavior.The cross-sectional shape of the molding channels 2 must not becontinuously conical (see the enlarged scale of FIG. 15 illustrating theventing geometry). The upper molding plate 4 in the drawing indicates acylindrical longitudinal section 15 and the lower molding plate 4 acylindrical longitudinal section 16 for the molding channel 2. Thecross-section of the two molding plates 4 tapers from the longitudinalsection 15 to the longitudinal section 16 of the molding channel 2 by afew μm to produce a weak step at this point. At this step region,venting takes place via the gap 13 between the two molding plates, whichmap into a widening 14. During release from the mold, these unnoticeablesteps are not visible and produce slight conicity along the entirelength of the bristle. The longitudinal sections 15, 16 in theindividual molding plates 4 can be produced through simple drilling.Alternatively, the longitudinal sections of the individual moldingplates can have identical diameters to produce a cylindrical bristle.More distinct diameter changes produce stepped bristles.

Conical bristles are technically advantageous for injection molding andfor removal from the mold. The smallest cross-section at the bristle endcools more rapidly than the subsequent regions of the bristle towardsthe root region and the step-by-step release from the bristle end to thebristle root follows the temperature gradient in the bristle.

The molding plates 4 have a thickness of a few millimeters. It maycorrespond approximately to three to fifteen times the diameter of themolding channel 2 so that extremely precise drilling of the longitudinalsections in the individual molding plates is possible. Since they arekept adjacent to one another under the closing pressure of theinjection-molding machine, even these thin molding plates maintain theirdimensions and shape, despite the high injection pressure. The lowthickness also ensures good thermal dissipation, since the moldingplates are evenly insulated by the venting gaps. They are easy to coolfor the same reason, e.g. using external coolants, which can beparticularly effective when the mold is closed, and also during the timebetween opening and renewed closing. Effective cooling already occursvia the surrounding air due to exposure of the molding plates and inconsequence of their small thickness. Alternatively, the cooling meansmay be integrated in or between the molding plates. Finally, the minimalloading under injection pressure permits production of the moldingplates from materials having good thermal conductivity with lessstringent mechanical strength properties than steel or the like.

The influence of effective cooling on the molecular structure of thebristles has already been discussed above.

FIG. 16 also schematically shows an injection mold 1 which consists oflayered molding plates 4, wherein the molding plate on the injectionside does not have widened cross-sections. In contrast to theabove-described embodiments, the molding plates 4 are divided into twogroups 17, 18 (see FIG. 17) wherein each group comprises at least onemolding plate which can be transversely displaced (indicated in FIGS. 17to 20 with double arrows 19, 20.)

The transversely displaceable molding plates cooperate with theneighboring molding plates to clamp the blanks 21, which, in thisembodiment, only form one portion (longitudinal section) of the finalbristle. The blank 21 is injected from a thermoplastic polymer withinjection parameters matched to this longitudinal section of thefinished bristle. After the injection cycle, at least one displaceableplate of the group 18 of molding plates 4 (FIG. 17) is brought into aclamping position and the blanks 21 are carried along when the group 18is removed to be thereby partially released from the molding plates 4 ofthe group 17 on the injection side and free a predetermined longitudinalsection 22 of the molding channels in the molding plates 4 of the group17. At the end of the blank 21, profilings may be optionally formed asindicated in the drawing. After withdrawal of the molding plates 4 ofthe group 18, the displaceable molding plate in the group 17 is broughtinto the clamping position and the exposed longitudinal sections 22 aresubsequently filled with a molten polymer mass, which consists ofanother polymer or a polymer with other additives. The longitudinalsections 23 of the bristle which are formed thereby connect to theblanks 21 through material bonding and/or positive locking.Subsequently, the displaceable forming plate in group 17 is returned toits initial position and the blanks 21 with molded-on longitudinalsections 23 are again partially withdrawn from the molding channels ofthe group 17 when the clamping means is closed to expose longitudinalsections 24 in the molding channels. In a further injection moldingcycle, the longitudinal sections 24 are filled with a further moltenpolymer mass with optionally further differentiated properties tofinally obtain bristles 27 having three regions (sections 21, 23 and 25)for different mechanical strength properties and/or different usageproperties along the bristle length. In particular, the region 21, whichencloses the bristle end, can serve as wear display to show the degreeof wear of the bristle. Final release of the bristles from the mold iscarried out as described above.

FIGS. 21 to 24 also show an injection mold 1 (FIG. 21) which consists oftwo groups 17, 18 of molding plates 4 each of which has at least onetransversely displaceable molding plate to form a clamping means. Incontrast to the above-described embodiment, the molding plate 5 on theinjection side has widenings, which taper towards the molding channel.

The molding plate 6 forming the bristle ends has blind holes 28, 29 and30 of different depths with dome-shaped hole bottoms such that aplurality of bristles of different lengths can be produced whose endslie on a curved envelope surface.

In the embodiment of FIGS. 21 to 24, bristles are injected sequentiallywith two different regions 31, 32 wherein the region 31 has an extendedbristle root 33. The multiple-section bristles 34 (FIG. 22) injected inthis fashion are subsequently removed from the mold at their ends byremoving the molding plate 6 forming the bristle ends and—optionallywith delay—removing the molding plates 4 of the group 18 (FIG. 22).Subsequently, at least one-transversely displaceable molding plate inthe group 18 is brought into a clamping position and the entire group18, optionally together with the terminal molding plate 6, is displacedin the opposite direction so that the part of the region 31 of thebristles 34 including the root region 33 project past the molding plate5 at the injection side. Subsequently, the injection mold 1 (FIG. 23) isconnected to a further injection mold 35 with a mold cavity 36 intowhich a molten polymer mass is injected with which the root regions 23and the longitudinal sections of the regions 31 which project into thecavity 36 are injected. The mold cavity 36 may be formed so that itdefines an intermediate support for the bristles or a complete brushbody in which the bristle ends are embedded without gaps so that theycannot be pulled out.

In a modification of this embodiment, the molding channels 2 of theinjection mold 1 of FIG. 21 can also be completely filled with onesingle molten polymer mass and, as shown in FIGS. 22 and 23, their rootregions can be exposed together with the adjacent longitudinal sectionsfor injection with the support-forming molten polymer mass (FIG. 24).

In a further modification, the bristles which are injected according toFIGS. 21 to 23 and released at their mounting-side ends can becompletely released from the mold through removing the molding plate 6forming the ends and the major part of the subsequent molding plates 4while being held by a few, at least three, molding plates, e.g. theinjection-side molding plate 5 and the two subsequent molding plates oneof which can be transversely displaced to form a clamping means. Thesemolding plates, which serve as a transport holder, can be transportedtogether with the bristles into another injection molding station inwhich they are brought into connection with the injection mold 35 whilesimultaneously providing a new set of molding plates with injection-sidemolding plate 5 to complete the injection mold 1. This transport holdercan move the bristles into the second injection molding station and alsocontinue transport into other processing stations.

FIGS. 25 and 26 show part of an injection mold 1 with molding plates 4and 5 after production of the bristles and removal of at least the lastmolding plate 6 (not shown). Replacing the latter, a flat thrust plate39 is moved in front of the released ends with which the bristles 38 aredisplaced in the molding channels of the remaining molding plates untiltheir root region 37, and optionally an adjoining longitudinal section,project past the injection-side molding plate 5 or into the mold cavity36 of the further injection mold 35 and are injected with a moltenpolymer mass to form a bristle support or a brush body.

FIGS. 27 and 28 show an embodiment with which, after production of thebristles 38 as described with reference to FIGS. 25 and 26, instead ofthe flat thrust plate 39, a thrust plate 40, which has cam-likeprojections 41 and 42 of different heights, is moved in front of thereleased bristle ends. When the thrust plate 40 has been moved towardsthe molding plates 4, the bristles are displaced along the thrust pathto different depths within the molding channels so that their rootregion 37 projects into the mold cavity 36 of the injection mold 35 todifferent depths and the bristle ends lie on a curved envelope surfaceafter injection and removal of the thrust plate 40 and molding plates 4and 5.

FIGS. 29 and 30 show an embodiment which differs from that of FIGS. 25and 26 only in that the bristles 38 are interconnected in the region ofthe injection-side molding plate 5 via a connection 43 in the form ofbars, grids or the like and project with the connection 43 and thesubsequent longitudinal sections of the bristles 38 into the cavity 36of the injection mold 35 after displacement via the thrust plate 39.

A smaller group of molding plates 4, preferably including theinjection-side molding plate 5 and with at least one molding plate 4which can be transversely displaced to act as clamping means, may serveas transport holder for transferring the bristles into further injectionmolding stations, processing stations or the like.

The layered structure of the injection mold from a plurality of moldingplates and the thereby possible sectional removal from the mold and theincrease in the modulus of elasticity and tensile strength obtained bythe inventive method parameters of injection pressure and flow speed inthe molding channel permit production of bristles whose central axis isnot in the direction of release from the mold. FIGS. 31 and 32 showexamples thereof. FIG. 31 shows a part of an injection mold withslanting molding channels 44, 45 that are inclined towards each other inthis embodiment. In addition to or alternatively, the injection mold 1may have wavy, curved molding channels 46 or molding channels 47 withseveral bends so that correspondingly formed bristles are produced whichcan be injected in a composite action via a connection 48. For releasefrom the mold, the molding plates 4 and 6 are removed, starting withmolding plate 6, and the bristles are released in sections without beingdeformed due to their high bending elasticity and the small releaselength.

The bristles may be fabricated into a brush after separation of theconnection, individually or in groups or together with the connection 48through injection around it or through other conventional thermal ormechanical connection methods.

In the embodiment of FIG. 32, the injection mold 1 has layered moldingplates 4 and two end molding plates 49, 50 that form distinctivelybranched bristle ends. The injection-molded bristles 51 each havefinger-like bristle ends 52 which can be easily removed from the molddue to the thin molding plates and the increased stability of thebristles.

The molding plates 6 or 49, 50 which form the bristle ends can be madefrom a sintered metal, in particular, for distinctly branched bristleends which also provides additional venting in this region toeffectively prevent trapping of air. The molding plates 4 can, ofcourse, also be made from such sintered metals to support venting of themolding channels. Micro-roughnesses which exist e.g. in sintered metalsor which can be produced through surface treatment of the moldingchannel produce corresponding roughnesses in the micro region on thesurface of the finished bristle which have a moisture repellant “Lotus”effect during use of the bristle.

FIG. 33 shows one individual bristle 53 which can be used in particularfor hygiene brushes, e.g. toothbrushes, cleaning brushes in the medicaland hospital fields or as cleaning or application brushes in the foodindustry. Suitable setting of injection pressure and flow velocity (corespeed) in the bristle-molding channel permits optimum adaptation of thebending behavior of the bristle along its length to the respectivepurpose of use, the bristles having an average diameter of 0.3 to 3 mm.They can widen like a trumpet in the root region 54 to obtain arelatively bending-resistant shoulder, which also forms a smoothtransition to the surface of the brush body 55. This entire gap-freeregion, the stem base and the actual stem of the brush 53 and thebristle end 56, which is uniformly rounded in the present case, can beproduced with injection molding technology to have smooth walls or wallswith micro-roughnesses to prevent occurrence of unwanted roughnesses andsoiling. Due to these properties, the brushes having bristles of thistype can also be easily cleaned and/or disinfected after use since nopockets, gaps or the like are present. Bristles having this shape andthe properties designed for the application can neither be produced byextrusion nor by injection molding methods known to date.

FIG. 34 shows two neighboring bristles 57, which are combined at theirtrumpet-like rounded root region 58 via a connection indicated with 59.Using the inventive method, the bristles 57 with the connection 59 canbe disposed at a slight separation from each other, which can, moreover,be optimally adjusted to the respective purpose of application. Thebristles 57 can be positioned very closely to prevent moisture, dirt orbacteria deposits or remnants following rinsing.

FIG. 35 shows a view of and FIG. 36 a top view onto a bristle 60produced according to the inventive method, which merges like a trumpetinto the bristle support surface 62 in the root region 61 and has a stem63 with relatively high bending strength and an effective region 64 withprofiled shape. In this embodiment, the effective region 64 has across-shaped cross-section, which gradually merges 65 into the stemregion. The cross-shaped cross-section of the effective region 64 formsbrushing edges, which become effective under strong loading of the brushand bending of the effective region. With reduced pressure, this effectoccurs at the rounded bristle end 66, which also has a cross-shapedprofile. The bristle end 66 can moreover penetrate into corners, gapsand furrows for cleaning it. The same effects can be obtained with otherpolygonal cross-sectional shapes.

1. A method for producing a bristle from a thermoplastic polymer byinjection molding a molten polymer mass into a bristle molding channel,said channel having a predetermined length and a predetermined crosssectional shape along said length, the method comprising the steps of:(a) injecting the molten polymer mass into said channel under pressure,said pressure being selected in dependence on said cross sectional shapeof said channel, a ratio of a largest width of a cross section of saidchannel to said length of said channel being selected as less than orequal to 1:10, wherein said injection pressure is 2000 to 5000 bar(2×10⁵ kPa to 5×10⁵ kPa), and sufficient to provide a specific pressurein a bristle forming channel of more than 300 bar (0.3×10⁵ kPa); and b)venting said channel along said length during step a), wherein a shearflow is established with a core speed of approximately 1000 mm/s in acenter of molten polymer mass flow and with a large shearing effect dueto wall friction of the molten polymer mass under distinct longitudinalorientation of polymer molecules, at least in a portion of the moltenpolymer mass proximate a wall of said channel, said longitudinalorientation of the polymer molecules being maintained throughout saidlength of said channel.
 2. The method of claim 1, wherein said injectionpressure is set to support crystal seed formation between neighboringlongitudinally oriented molecular sections in dependence on said crosssectional shape and said length of said bristle-molding channel.
 3. Themethod of claim 1, wherein said bristle-molding channel is cooled. 4.The method of claim 1, wherein said bristle-molding channel is ventedtransverse to a flow direction of the molten polymer mass.
 5. The methodof claim 4, wherein said bristle-molding channel is vented in severalplanes disposed transverse to a flow direction of the molten polymermass.
 6. The method of claim 5, wherein said bristle-molding channel isvented along said length via planes disposed at approximately equaldistances.
 7. The method of claim 1, wherein said bristle-moldingchannel is vented of air displaced by flow pressure of the moltenpolymer mass.
 8. The method of claim 1, wherein said channel is ventedwith assistance of an external underpressure.
 9. The method of claim 1,wherein said cross section of said bristle-molding channel remainssubstantially constant, beginning at an injection side thereof.
 10. Themethod of claim 1, wherein said cross section of said bristle-moldingchannel tapers substantially continuously from an injection sidethereof.
 11. The method of claim 1, wherein the molten polymer mass isinjected into an inlet region which narrows like a nozzle towards saidbristle-molding channel to produce an extension flow.
 12. The method ofclaim 1, wherein said cross sectional shape of said bristle-moldingchannel has at least one discontinuity configured as a tapering in aflow direction of the molten polymer mass.
 13. The method of claim 1,wherein said cross section of said bristle-molding channel is selectedto have a maximum width of ≦3 mm.
 14. The method of claim 1, wherein aratio of a largest width of said channel to said length of said channelis selected to be ≦1:250.
 15. The method of claim 1, wherein the moltenpolymer mass is simultaneously injected into several neighboringbristle-molding channels thereby forming a corresponding number ofbristles.
 16. The method of claim 15, wherein, the molten polymer massis injected into neighboring bristle-molding channels whilesimultaneously forming a connection between at least two bristles. 17.The method of claim 15, wherein, after injection of the bristles, amolten polymer mass of another polymer is subsequently injected, therebyforming a connection between at least two bristles.
 18. The method ofclaim 15, wherein the molten polymer mass is injected to form a bristlesupport which connects at least two or more bristles.
 19. The method ofclaim 16, wherein the molten polymer mass is injected to form a bristlesupport which connects the bristles and forms a brush body.
 20. Themethod of claim 18, wherein at least one further molten polymer massfrom another polymer is injected onto said bristle support.
 21. Themethod of claim 15, wherein a number of bristles are injected withdifferent lengths.
 22. The method of claim 15, wherein a number ofbristles are injected with different cross sections.
 23. The method ofclaim 15, wherein a number of bristles are injected with a crosssectional shape which changes along their lengths.
 24. The method ofclaim 15, wherein a plurality of bristles are injected with parallelmutual orientation.
 25. The method of claim 15, wherein at least onepart of the bristles is injected in a non-parallel fashion.
 26. Themethod of claim 15, wherein bristles of a same geometry but differentbending elasticity (hardness) are produced through injection-molding ofdifferent molten polymer masses in same molding channels.
 27. The methodof claim 1, wherein the bristles are injected from a polymer or apolymer mixture which has reduced secondary binding forces in asolidified state.
 28. The method of claim 1, wherein the bristles areinjected from a polymer including additives which become active duringuse.